Denitrification and sulfate reduction in fractured coastal aquifers subjected to tides

Fractured coastal aquifers are particularly susceptible to saltwater intrusion (SWI), and the effects of fractures on SWI affect the fate of land-sourced and sea-sourced nutrients in coastal aquifers, but the reactivity of biogeochemical process in such aquifers remains largely unknown. A numerical variable-density groundwater flow and reactive transport model was used to evaluate the effects of fracture characteristics (vertical position of horizontal fracture, horizontal fracture length, vertical fracture length, and horizontal position of vertical fracture) on the reactivity of mixing-dependent and mixing-independent reactions (represented as denitrification and sulfate reduction, respectively) in fractured coastal aquifers. Simulation results show that denitrification removes up to 64%-92% of land-sourced nitrate, and sulfate reduction transform only about 5 parts per thousand of sea-sourced sulfate prior to discharge. Under the model settings and conditions tested, the simulation results indicate that the fracture characteristics of the aquifer are a major factor in regulating the behavior of both mixing-dependent and mixing-independent reactions. Horizontal fractures promote or attenuate denitrification and sulfate reduction primarily by affecting the area of the upper saline plume, and reactant solutes mixing. Vertical fractures primarily affect reactant solutes mixing to promote or attenuate denitrification and sulfate reduction. Mixingdependent reactivity increases with the size of the mixing zone and solute supply, while mixing-independent reactivity is mainly controlled by solute supply and area of the upper saline plume. These results have important implications for understanding biogeochemical processes in fractured coastal aquifers and for the management of coastal ecosystems.